The described embodiments relate generally to the field of conversion of direct current (DC) to alternating current (AC) and vice versa, and more specifically, relate to converters (inverters and rectifiers) that are multilevel.
Such power conversion equipment is particularly useful for renewable power generation systems such as wind and solar power generation systems. Generally a wind turbine includes a rotor that includes a rotatable hub assembly having multiple blades that transform wind energy into a mechanical rotational torque that drives one or more generators via the rotor. With the rapid growth of grid-connected renewable power generation systems, renewable power penetration into the power grid may have a significant impact on the grid voltage and frequency. It is desirable to regulate the voltage and frequency of the AC power at the output of the power generation system. In wind turbine embodiments, one or more power converters are coupled to the generator to convert the power to provide power with an appropriate frequency and voltage for the utility grid.
FIG. 1 shows a converter embodiment 10 which will be used to exemplify general multilevel concepts. Although an inverter is used as the basis for this discussion, the multilevel converter can be used in active rectifier, flexible AC transmission systems (FACTS), and other appropriate fields. The converter embodiment 10 includes a multilevel converter (inverter) 12, an AC load 14, a pulse-width modulation (PWM) signal generator 16, and a filter circuit 13 including inductors L, capacitors C, and resistors R. The converter 12 is used to convert the DC signals to AC signals according to the PWM control signals from the PWM signal generator 16. FIG. 2 shows inverter line-to-ground voltages for several values of n (herein, n is used to represent the number of voltage levels). According to the modulation process, the output of the converter 12 is an ideal sine-wave with switching harmonics. Increasing the voltage levels and modulating the multi-level signals with a PWM function results in a converter output voltage that more closely tracks the ideal sinusoidal output by reducing the undesired harmonics.
For achieving the conversion function of DC to AC (or AC to DC), some topology configurations of the power source converter are designed accordingly. One classic topology configuration of the power source converter is a cascaded H-bridge converter topology. FIG. 3 shows a phase of a classic cascaded H-bridge converter topology 30. The converter topology 30 is a single-phase structure of an m-level cascaded inverter. Each separate and electrically isolated DC source (SDCS) is connected to a single-phase two-leg full-bridge, or H-bridge inverter 32. Each H-bridge inverter 32 (inverter level) can generate three different voltage outputs, +Vdc, 0, and −Vdc by connecting the DC source to the AC output by different combinations of the four switches S1, S2, S3, and S4. To obtain +Vdc, switches S1 and S4 are turned on, whereas −Vdc can be obtained by turning on switches S2 and S3. By turning on S1 and S2 or S3 and S4, the output voltage is 0. The AC outputs of each of the different full-bridge inverter levels 32 are connected in series such that at any instant the synthesized voltage waveform is the sum of the inverter outputs. The number of output phase voltage levels m in a cascaded inverter is defined by m=2s+1, where s is the number of SDCSs. Even though the cascaded H-bridge converter topology 30 is successful in converting DC to AC (or AC to DC), this topology may become bulky and costly when the number of levels exceeds three, due to the large number of both active and passive components present in the circuits.
Therefore, it is desirable to provide a new multilevel power source converter topology configuration to at least reduce the number of the active components making it more economical, while maintaining a high efficiency and generating waveforms of high quality.